By just about anybody's math, Marty Kibiloski should not have been on the verge of a nervous breakdown. As a sub-15-minute 5-K road racer and a guy who had finished the Boston Marathon in 2:23, Kibiloski's endorphin-rich blood probably could have been bottled and sold as an antidepressant. But for some reason, all that running wasn't enough therapy for the then-45-year-old banking executive--the man was stressed, and it was taking a toll. "I would raise my voice at my kids for no reason," he says. "At work, I missed a flight because I was trying to pack too much into the day, and I even lost the deal."

That changed in 2005 when, on a trip to the Himalayas, Kibiloski took part in a 100-mile run on a trail that separates India and Nepal. "My focus on that run went from the destination to the journey," he says. "I began letting go of the outcome and focused on enjoying the experience." Almost overnight, Kibiloski's stress started to dissipate. "I wasn't caught up anymore with how much I needed to get done each day. I began enjoying life."

Any exercise causes the body's pituitary gland to release endorphins--a natural chemical that has opiate-like effects on the brain--which, a 2009 University of Vermont study showed, causes people to remain in a good mood for up to 12 hours after exercise. Running happens to be one of the better ways of firing up the pump. But for lots of road runners, the positive effects are often negated by honking horns and car exhaust.

"Trails just have a way of closing off the rest of the world and all of the chaos," says Dr. Jerry Lynch, a Boulder, Colorado-based psychologist and author. "Trail running is quiet and contemplative." Lynch even prescribes trail running to his patients who suffer from depression. "I've had several clients over the years who were depressed and taking medication and it wasn't working. I steered them toward trail running and they became more at peace with themselves and found joy."

There may be even more at work here. A 1996 study showed that negative ions--invisible air molecules released by trees that are known to increase oxygen flow to the brain--alleviate seasonal depression as effectively as Prozac or Zoloft.

It works for the Marathon Monks of Mount Hiei, Japan. These Buddhists trail run between 24 and 82 miles a day for 1,000 straight days in order to achieve enlightenment. "They're able to really get back to their animal nature and become more aware of life," says Lynch.

Trails are also at the center of what's happening at Shambhala Mountain Center in Boulder, Colorado, where Marty Kibiloski has gone from patient to teacher. Twice a year for five days, he instructs students in yoga, meditation, and running in a meditative state. "We also teach basic running skills like form and how to breathe correctly," he says. "Once you get it, it helps make your running more meditative. It just allows you to be more thoughtful."